Transmission of mouse hepatitis virus (MHV) was studied in a "natural" animal room setting using euthymic and athymic mice; and in unnatural settings where experimentally infected cage mates served as time-controlled donors. We have demonstrated that euthymic sentinel mice may not be as effective indicators of MHV infection in mouse populations as generally accepted because of slow, or low titre response under certain husbandry conditions. In a single companion study, athymic sentinels gave histologically predictive indications of incipient MHV at 7-11 weeks post placement, and fully developed MHV liver lesions at 13-17 weeks post placement. The nude sentinel concept must be approached cautiously because the long patent period of MHV infection in nude mice identifies them as contributors to the persistence of the virus within the colony. We have also shown that CD-1 weanling mice experimentally infected with a street strain of MHV shed virus between 12 and 18 days PI as measured by cohabitant seroconversion. It must be emphasized that these results are indicators for the CD-1 mouse only, and must be confirmed. We are presently repeating the euthymic sentinel study in a carefully controlled circumstance where sentinels are examined biweekly and/or monthly in known infected and noninfected rooms.